Cómo consultar datos de la base de datos utilizando valores de cadena de consulta

Debe verificar si $request tiene las claves

Prueba

public function index(Request $request)
{
    $query = Vehicle::query();
    
    if($request->has('condition')) {
        $query->where('condition', $request->condition);
    }

    if($request->has('make')){
        $query->where('make', $request->make);
    }
    
    if($request->has('model')) {
        $query->where('model', $request->model);
    }

    if($request->has('year')){
        $query->where('year', $request->year);
    }

    $vehicles = $query->paginate(20);

    $conditions = DB::table("conditions")->pluck("name","id");

    return view('cars',compact('conditions', 'vehicles'));

}